1. Field of the Invention
This invention relates to a gas sensor, and more particularly to an improvement of oxygen sensors which have been increasingly used as air-fuel-ratio sensors in streams of exhaust gas from internal combustion engines or the like. The invention is an outcome of research and development efforts for improving the response of an air-fuel-ratio sensor while eliminating detrimental cooling of the sensitive element thereof by the stream of the gas being measured, and the air-fuel-ratio sensor of the invention can be applied, but not limited, to the control of fuel supply, liquid or gas, to combustion means such as automobile engines.
2. Description of the Prior Art
The applicants disclosed in their Japanese Patent Laying-open Publication No. 153,155/1983 an oxygen sensor comprising an oxygen pump element having an oxygen-ion-conductive solid electrolyte plate with a pair of aligned electrodes secured to the opposite surfaces thereof and an oxygen concentration cell element having an identical structure with that of the oxygen pump element, the two elements being disposed parallel to each other so that one electrode of the oxygen pump element faces one of the paired electrodes of the oxygen concentration cell element with a spacing therebetween, so as to define a diffusion-restricting narrow gap by such spacing.
With the oxygen sensor of the above structure, the oxygen pump element pumps out oxygen from the narrow gap to the outside of the oxygen sensor at a rate depending on the current through the two electrodes thereof. As the oxygen density in the narrow gap is reduced by such pumping, oxygen diffuses into the narrow gap from the outside at a rate depending on the diffusion resistance of the narrow gas and the difference of oxygen concentration between the inside and the outside of the narrow gap. When the inflow of oxygen toward the gap balances the oxygen extraction by the oxygen pump, the oxygen concentration in the narrow gap is stabilized. Thus, the oxygen concentration of the gas outside the oxygen sensor can be determined either by the voltage across the oxygen concentration cell element or the current through the oxygen pump element, and the response of such oxygen sensor can be made quick.
However, the above oxygen sensor has a shortcoming in that when it is disposed in a turbulent or agitated flow of the gas being measured, such as in an exhaust gas tube from an internal combustion engine, the performance of the oxygen sensor is considerably affected by such agitated stream of the gas.
To prevent such influence of the agitated stream of the gas being measured, it has been suggested to cover the sensitive element of the oxygen sensor by a porous cylindrical protector. The inventors have found that excessive cooling of the sensitive element by the gas stream is hardly prevented by a simple porous cylindrical protector. Especially, when a protector with a high porosity is used to improve the response characteristics, the detrimental excess cooling of the sensitive element has been inevitably enhanced.